Researchers have become increasingly aware of the potential for nuclear magnetic resonance (NMR) imaging, also called magnetic resonance imaging (MRI), for materials science applications. NMR uniquely allows noninvasive/nondestructive mapping of the internal chemical and physical properties of materials and provides quantitative information on the chemical microstructure of materials. (See further, "NMR Imaging of Materials, Analytical Chemistry, Vol. 61, No. 1, page 23a).
The current invention contemplates use of NMR imaging for any nuclei normally NMR imaged, including proton, carbon, sodium and phosphorus, in a variety of materials including polymers and geologic materials, such as coal, and is particularly useful in imaging structural ceramics.
Hydrogen nuclei (protons), which are present in organic binders of green ceramic bodies, are the most sensitive NMR-active nuclei that can be used for MRI studies. With proper set up of an imaging experiment (choosing pulse sequences), NMR signal intensity (gray scale levels) can be made proportional to the amount of organics present in a local volume of interest (voxels). Like x-ray computed tomography, MRI can be used to provide two dimensional tomographic images of selected slices and as a quantitative technique for determining spatial distribution of organics within a green body.
Medical MRI systems based on solutions NMR (e.g. using water molecules abundant in biological subjects) are inadequate for the imaging of organics within a green body primarily because of differences between the line widths of NMR spectra. For example, the line width of proton spectra from organics in green ceramic materials is about 2500 Hz, compared to a few Hz in biological systems. Because linear gradient fields are used in NMR imaging to frequency-label spatial positions, the gradient strength required to resolve two positions in space must be enough to ensure that the difference in resonance frequencies between these two positions is greater than the line widths of the resonances. The imaging of ceramics with a spatial resolution of 100 .mu.m, for example, would require a gradient strength of 50 G/cm, which is beyond that normally found in medical systems.
Another difference is in imaging technique. While spin-warp imaging is used in medical MRI systems, back projection is the method of choice for materials with short spin-spin relaxation time, T.sub.2 (large line widths correspond to short T.sub.2). This method allows NMR response (FID) to be detected immediately after RF excitation, thus preserving maximum signal intensity. Back-projection, however, poses more stringent specifications on probe design, requiring (1) high uniform gradient and RF fields, (2) well-balanced gradient fields between orthogonal axes, and (3) strict alignment of static, gradient, and RF fields with respect to the center of the sample space. Also, the RF bandwidth must be great enough to span the entire range of frequencies produced by the gradient fields.
Modern NMR spectrometers and imaging units typically require some degree of digital control over a large number of subcomponents including RF amplifiers, RF receivers, frequency synthesizers, magnetic field gradient controllers, attenuators, filters and phase shifters. Demanding experimental sequences may require 100 nsec resolution, submicrosecond control, and output (perhaps thousands of commands over several seconds) to more than 250 digital control lines.
State of the art pulse programmers incorporate a pulse sequence to be expressed when the experiment is performed. The present invention provides an apparatus which provides for easy implementation with a variety of commonly available host computers and incorporates digital control ability to an expandable number of control/data acquisition lines, permitting on-site programming of complex, high speed pulse sequences with a large number of unique instructions for better resolution in pulse shaping.
It is therefore a primary object of this invention to provide a solid state NMR imaging system with high gradient field strength and the ability to narrow line widths.
In the accomplishment of the foregoing object, it is another important object of this invention to provide an NMR imaging system using back projection techniques while providing high uniform gradient and RF fields, well-balanced gradients fields between orthogonal axes, and strict alignment of static, gradient, and RF fields with respect to the center of the sample.
It is another important object of this invention to provide a flexible solid state NMR imaging accessory for operation with conventional wide-bore solid-state NMR spectrometers.
It is a further object of this invention to present an NMR imaging probe which permits use of alternative RF coils for varying sample sizes for increased efficiency and signal-to-noise ratio.
Finally, it is an object of the present invention to present a PC-based pulse programmer which is programmable to synchronize control pulses for gradient coils with a spectrometer radio frequency and which is usable with commonly available heat computers of choice.